The present invention relates generally to the manufacture of absorbent creped paper products including both cube embossing and substantially cross-machine direction perforate embossing. In one embodiment, the products are made from furnish incorporating at least about 15% bleached chemithermomechanical pulp (BCTMP).
Embossing is the act of mechanically working a substrate, such as a web or a cellulosic web, to cause the substrate to conform under pressure to the depths and contours of a patterned embossing roll. Generally the web is passed between a pair of embossing rolls that, under pressure, form contours within the surface of the web. During an embossing process, the roll pattern is imparted onto the web at a certain pressure and/or penetration. In perforate embossing the embossing elements are configured such that at least a portion of the web located between the embossing elements is perforated. As used herein, “perforated” refers to the existence of at least one of (1) a macro-scale through aperture in the web, (2) when a macro-scale through aperture does not exist, at least incipient tearing such as would increase the transmittivity of light through a small region of the web, or (3) a decrease the machine direction strength of a web by at least 15% for a given range of embossing depths.
Embossing is commonly used to modify the properties of a web to make a final product produced from that web more appealing to the consumer. For example, embossing a web can improve the softness, absorbency, and bulk of a final product. Embossing can also be used to impart an appealing pattern to a final product.
Embossing is carried out by passing a web between two or more embossing rolls, at least one of which carries the desired emboss pattern. Known embossing configurations include rigid-to-resilient embossing and rigid-to-rigid embossing.
In a rigid-to-resilient embossing system, a single or multi-ply substrate is passed through a nip formed between a first roll, whose substantially rigid surface contains the embossing pattern as a multiplicity of protuberances and/or depressions arranged in an aesthetically-pleasing manner, and a second roll, whose substantially resilient surface can be either smooth or also contain a multiplicity of protuberances and/or depressions that may cooperate with the rigid surfaced patterned roll. Commonly, rigid rolls are formed with a steel body which is either directly engraved upon or which can contain a hard rubber cover or other suitable rigid surface (directly coated or sleeved) upon which the embossing pattern is formed by any convenient method such as, for example, laser engraving. The resilient roll may consist of a steel core provided with a resilient surface, such as being directly covered or sleeved with a resilient material such as rubber or other suitable polymer. The resilient surface may be either smooth or engraved with a pattern. The pattern on the resilient roll may be either a mated or a non-mated pattern with respect to the pattern carried on the rigid roll.
In a rigid-to-rigid embossing process, a single-ply or multi-ply substrate is passed through a nip formed between two substantially rigid rolls. The surfaces of both rolls contain the pattern to be embossed as a multiplicity of protuberances and/or depressions arranged into an aesthetically-pleasing manner where the protuberances and/or depressions in the second roll may cooperate with those patterned in the first rigid roll. The first rigid roll may be formed, for example, with a steel body which is either directly engraved upon or which can contain a hard rubber cover or other suitable rigid surface (directly coated or sleeved) upon which the embossing pattern is engraved by any conventional method, such as laser engraving. The second rigid roll can be formed with a steel body or can contain a hard rubber cover or other suitable rigid surface (directly coated or sleeved) upon which any convenient pattern, such as a matching or mated pattern, is conventionally engraved or laser-engraved. In perforate embossing, a rigid-to-rigid embossing system is typically used; however, a rigid-resilient configuration may also be used for perforate embossing.
When substantially rectangular embossing elements have been employed in perforate embossing, the embossing elements on the embossing rolls have generally been oriented so that the long direction axis, i.e., the major axis, of the elements extend only in the machine direction. That is, the major axis of the elements is oriented to correspond to the direction of the running web being embossed. These elements are referred to as machine direction elements. As a result, the elements produce perforations which extend primarily in the machine direction and undesirably decrease the strength of the web in the cross-machine direction. This orientation improves absorbency and softness but can degrade, i.e., reduce the strength of, the web primarily in the cross-machine direction while less significantly degrading the strength of the web in the machine direction. As a result, the tensile strength of the web in the cross-machine direction is reduced relatively more, on a percentage basis, than that of the machine direction. In addition, the cross-machine direction strength of the base sheet is typically less than that of the machine direction strength. As a result, by embossing with machine direction elements only, the cross-machine direction strength is even further weakened and, accordingly, because the finished product will fail in the weakest direction, the product will be more likely to fail when stressed in the cross-machine direction.
Cross-machine direction tensile strength can be associated with consumer preference for paper toweling. In particular, consumers prefer a strong towel, of which cross-machine direction and machine direction strength are two components. Because an un-embossed base sheet is typically much stronger in the machine direction than the cross-machine direction, a process is desired which results in improved softness without sustaining excessive losses in cross-machine direction tensile strength.
The present invention addresses at least the above described problem by providing at least one embossing pattern, wherein at least a portion of the elements are oriented to provide perforating nips which are substantially in the cross-machine direction and are configured to perforate emboss (pert-emboss) the web, thereby preserving more of the cross-machine direction strength. In addition, the present invention may also provide at least two embossing rolls, where the embossing elements on at least one embossing roll are configured to impart an embossing pattern on the web, and where the embossing pattern includes elongated embosses in one or both of the machine direction and the cross-machine direction.
Additionally, in view of the rising costs of virgin fibers, the use of recycled cellulosic furnish to make towel and tissue products is often desirable, especially for facilities that produce large volumes of absorbent products. Products made from recycle furnish, however, tend to be relatively stiff, having relatively high tensile strengths and relatively low bulk leading to poor absorbency and softness properties. Moreover, these products tend to have relatively low wet/dry strength ratios. Various methods have been employed to increase the bulk and softness of products made from recycle furnish, including the use of softeners, debonders, and the like, the use of anfractuous fibers, and/or the use of new processing techniques. Many of these methods require significant capital investment and cannot be readily adapted to existing production capacity, such as conventional wet-press (CWP) paper machines with Yankee dryers.
There is disclosed in U.S. Pat. No. 5,607,551, which is incorporated herein by reference in its entirety, through-air-dried (TAD) tissues made without the use of a Yankee dryer. The typical Yankee functions of building machine direction and cross-machine direction stretch are replaced by a wet end rush transfer and the through-air-drying fabric design, respectively. According to the '551 patent, it is particularly advantageous to form the tissue with chemi-mechanically treated fibers in at least one layer. Resulting tissues are reported to have high bulk and low stiffness. Furnishes enumerated in connection with the '551 patent process include virgin softwood and hardwood as well as secondary or recycle fibers (see col. 4, lines 28-31). In the '551 patent it is further taught to incorporate high-lignin content fibers such as groundwood, thermomechanical pulp, chemimechanical pulp, and bleached chemithermomechanical pulp. Generally these pulps have lignin contents of about 15 percent or greater, whereas chemical pulps (Kraft and sulfite) are low yield pulps having a lignin content of about 5 percent or less. The high-lignin fibers are subjected to a dispersing treatment in a disperser in order to introduce curl into the fibers. The temperature of the fiber suspension during dispersion may be about 140° F. or greater. In one embodiment, the temperature may be about 150° F. or greater and, in yet another embodiment, the temperature may be about 210° F. or greater. The upper limit on the temperature may be dictated by whether or not the apparatus is pressurized, since the aqueous fiber suspensions within an apparatus operating at atmospheric pressure should not be heated above the boiling point of water.
It is believed that the degree of permanency of the curl is greatly impacted by the amount of lignin in the fibers being subjected to the dispersing process, with greater effects being attainable for fibers having higher lignin content (see col. 5, lines 43 and following). Lignin-rich, high coarseness, generally tubular fibers are further described in U.S. Pat. Nos. 6,254,725, 6,074,527, 6,287,422, 6,162,961, 5,932,068, 5,772,845, and 5,656,132, each of which is incorporated herein by reference in its entirety. The so-called uncreped, through-air-dried process of the '551 patent requires a relatively high capital investment and is expensive to operate inasmuch as thermal dewatering of the web is energy intensive and is sensitive to fiber composition.
Commercial success has also been achieved in connection with U.S. Pat. No. 5,690,788, which is incorporated herein by reference in its entirety. In accordance with the '788 patent, there is provided biaxially undulatory single ply and multiply tissues, single ply and multiply towels, single ply and multiply napkins, and other personal care and cleaning products, as well as creping blades and processes for the manufacture for such paper products. Generally speaking, there is provided in accordance with the '788 patent a creping blade provided with an undulatory rake surface having trough-shaped serrulations in the rake surface of the blade. The undulatory creping blade has a multiplicity of alternating serrulated sections of either uniform depth or a multiplicity of arrays of serrulations having non-uniform depth. The blade is operative to impart a biaxially undulatory structure to the creped web such that the product exhibits increased absorbency and softness with a variety of furnishes. Specifically disclosed are conventional furnishes such as softwood, hardwood, recycle, mechanical pulps (including thermo-mechanical and chemithermomechanical pulp), anfractuous fibers, and combinations of these (see col. 20, line 41 and following). Example 20 of the '788 patent notes the properties obtained when using the undulatory blade in the manufacture of towels including up to 30 percent anfractuous fiber high bulk additive (HBA). HBA is a commercially available softwood Kraft pulp sold by Weyerhauser Corporation that has been rendered anfractuous by physically and chemically treating the pulp such that the fibers have permanent kinks and curls imparted to them. Inclusion of the HBA fibers into the base sheet will serve to improve the sheet's bulk and absorbency.
Despite many advances in the art, there is an ever present need for further improvements to products which incorporate cellulosic fiber such as recycled fiber, especially those improvements that do so on a cost-effective basis in terms of required capital and operating costs. It has also been found that there is a benefit between the use of an undulatory creping blade and the incorporation of certain high yield fibers into a web.
As embodied and broadly described herein, the invention includes an embossing system for embossing at least a portion of a web comprising a first roll and at least a second roll, the first roll and second roll defining a first nip for embossing the web, wherein at least one of the first roll and the second roll may include elongated embossing elements extending substantially in the machine direction and at least one of the first roll and the second roll may include perforate embossing elements extending substantially in the cross-machine direction, and wherein the embossing elements are capable of imparting a perforate pattern and/or a cube embossing pattern on the web. The embossing elements extending substantially in the machine direction and the perforate embossing elements extending substantially in the cross-machine direction may be provided on the same or both of the first and the second embossing rolls. In one embodiment, the web may be a cellulosic fibrous web, wherein at least about 15% by weight of the fiber, based on the weight of the cellulosic fiber in the furnish, is lignin-rich, high coarseness fiber having generally tubular fiber configuration, as well as an average fiber length of at least about 2 mm and a coarseness of at least about 20 mg/100 m. In another embodiment, the web may be creped with an undulatory creping blade. In a further embodiment, both the first and second rolls include elongated mated embossing elements extending substantially in the machine direction. In yet another embodiment, the elongated embossing elements extending substantially in the machine direction are capable of imparting a cube embossing pattern to the web, and the perforate embossing elements extending substantially in the cross-machine direction are capable of imparting a perforate pattern to the web.
Another embodiment of the invention includes a method of embossing at least a portion of a web, including providing a first roll and providing at least a second roll, the first roll and the second roll defining a first nip, providing a cellulosic fibrous web to be embossed, and passing the web between the first nip, wherein at least one of the first roll and the second roll has elongated embossing elements extending substantially in the machine direction and/or the cross-machine direction and optionally at least one of the first roll and the second roll has perforate embossing elements, that may or may not be elongated, extending substantially in the cross-machine direction, and wherein the elongated embossing elements impart a cube embossing pattern on the web. In one embodiment, both of the substantially machine direction embossing elements and the substantially cross-machine direction perforate embossing elements are on the same roll. In another embodiment, both the first and second rolls include elongated mated embossing elements substantially in the machine direction and/or the cross-machine direction. In a further embodiment, the elongated embossing elements extending substantially in the machine direction and/or the cross-machine direction are capable of imparting a cube emboss pattern to the web, and the perforate embossing elements, that are not elongated, extending substantially in the cross-machine direction are capable of imparting a perforate emboss to the web. In yet a further embodiment, at least one of the first roll and the second roll have both elongated embossing elements extending substantially in the machine direction and elongated embossing elements extending substantially in the cross-machine direction that are capable of imparting a cube emboss pattern to the web, and no perforate embossing elements extending substantially in the cross-machine direction are capable of imparting a perforate emboss to the web. In still a further embodiment, at least one of the first roll and the second roll have both elongated embossing elements extending substantially in the machine direction and elongated embossing elements extending substantially in the cross-machine direction that are capable of imparting a cube emboss pattern to the web, and perforate embossing elements extending substantially in the cross-machine direction that are capable of imparting a perforate emboss to the web.
In another embodiment of the present invention, a first roll and a second roll are provided, the first roll and the second roll defining a first nip for embossing a web, wherein at least one of the first roll or the second roll includes elongated embossing elements substantially extending in the machine direction, wherein at least one of the first roll and the second roll includes elongated embossing elements extending substantially in the cross-machine direction, and wherein at least one of the first and the second roll includes substantially cross-machine direction embossing elements. In one embodiment, the substantially cross-machine direction embossing elements are perforate embossing elements. In another embodiment, each of the elongated substantially machine direction embossing elements, the elongated substantially cross-machine direction embossing elements, and the substantially cross-machine direction elements may be on one roll. In a further embodiment, both the first roll and the second roll include elongated mated embossing elements extending substantially in the machine direction and/or the cross-machine direction. In yet another embodiment, the elongated embossing elements extending substantially in the machine direction and the elongated embossing elements extending substantially in the cross-machine direction are capable of imparting a cube emboss pattern to the web, and the perforate embossing elements, that are not elongated, extending substantially in the cross-machine direction are capable of imparting a perforate emboss to the web.
The accompanying drawings, which are incorporated herein and constitute a part of this specification, illustrate an embodiment of the invention, and, together with the description, serve to explain the principles of the invention. Further advantages of the invention will be set forth in part in the description which follows and in part will be apparent from the description or may be learned by practice of the invention. The advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.